The present invention relates to hydrostatic power steering systems, and more particularly, to fluid controllers for use therein.
A typical fluid controller for use in a power steering system includes a housing which defines various fluid ports, and further includes a fluid meter and valving, operable in response to an input, such as the rotation of a vehicle steering wheel. The typical fluid controller also includes an arrangement for imparting follow-up movement to the valving in response to the flow of fluid through the valving and the fluid meter, during a normal steering operation. The flow of fluid through the controller valving is generally proportional to the rate at which the steering wheel is rotated.
A typical fluid controller of the type to which the present invention relates is illustrated and described in U.S. Pat. Nos. 4,804,016; 5,042,250; and 5,620,026, all of which are assigned to the assignee of the present invention and are incorporated herein by reference. In the conventional fluid controller shown in the above-cited patents, and sold commercially by the assignee of the present invention, the arrangement for imparting follow-up movement to the valving has included a drive shaft having a rearward end in splined engagement with the orbital and rotatable star of the fluid meter. The drive shaft has also included a forward end which is bifurcated, i.e., it defines an open, generally U-shaped portion disposed about a diametrally oriented pin member. The pin member passes through circumferentially elongated openings in the inner valve member (spool valve), and is received in a close fit relationship within holes defined by the outer valve member (sleeve valve). Therefore, in the conventional arrangement for imparting follow-up movement, the pin remains fixed relative to a diameter through the sleeve valve, while the surrounding, bifurcated forward end moves ("wobbles") relative to the pin as the rearward end of the drive shaft orbits and rotates.
The follow-up arrangement described above has been in use commercially for many years, and has been generally satisfactory, especially as long as the fluid pressures being controlled are relatively low, e.g., in the range of 1000 psi to 2000 psi. However, one disadvantage of the prior art follow-up arrangement relates to the bifurcated end of the drive shaft. As is understood by those skilled in the art, because there is relatively little area of engagement between the bifurcated end of the drive shaft and the diametral pin, that portion of the drive shaft has had to be made larger in the diametral direction, in order to provide reasonably satisfactory area of engagement of the drive shaft and the pin.
Unfortunately, even though the drive shaft has typically been relatively compact (i.e., having a small diameter) over most of its axial length, the requirement for the enlarged diametral dimension adjacent the pin has required that the inside diameter of the spool valve be larger than is really desirable, simply to accommodate the forward end of the drive shaft.
With the minimum inside diameter of the spool valve being thus determined, the designer of the fluid controller has, prior to the present invention, had two options. Either the radial thickness of the spool valve can be decreased, in which case the spool valve is more likely to be deflected inward by relatively higher fluid pressures, or the outside diameter of the spool valve can be made larger. In the latter case, the diameter of the sleeve valve will also have to be made larger, and as is well known to those skilled in the art, sealing of cylindrical clearances becomes more difficult and expensive as the diameter of the cylindrical clearance increases.
Another disadvantage of the prior art follow-up arrangement is the manufacturing cost of providing both the bifurcated forward end and a set of straight, internal splines within the central opening of the star, and a set of crowned, external splines about the rearward end of the drive shaft. Such splines typically require a relatively more sophisticated manufacturing operation, such as hobbing.